1. Field of the Invention
The present invention is generally related to power overload control in communication systems.
2. Description of the Related Art
Third generation (3G) wireless communication systems such as Code-Division-Multiple-Access (CDMA) networks and Universal Mobile Telecommunication Systems (UMTS) typically may include a plurality of mobiles having transceivers communicating with transceivers of serving base stations. Each mobile transceiver may include a transmitter and a receiver which communicate with a corresponding base station receiver or transmitter via one or more links. A link typically may comprise a plurality of communication channels such as signalling channels and traffic channels, for example. Traffic channels are communication channels through which users convey (i.e., transmit and/or receive) user information. Signalling channels may be used by the system equipment to convey signaling information used to manage, operate and otherwise control the system. The system equipment, which may be typically owned, maintained and operated by a service provider, may include various known radio and processing equipment used in communication systems. The system equipment along with user equipment (UE), e.g., mobiles, generate and receive the signaling information.
Communication signals transmitted and received via communication links may often be distorted by various anomalies that exist in the communication channels. These channel anomalies may cause the signals to be received erroneously. For example, channel anomalies such as path loss, Rayleigh fading, frequency translation and phase jitter may often cause the signals to lose power, so that a signal is received at a significantly lower power level than it was transmitted. As a result, signals adversely affected by channel anomalies may often be received with errors. One way of preventing errors from occurring, or at least to reduce the likelihood of errors occurring, is by applying power control techniques to these communication systems.
In general, a power control algorithm may be performed at a base station. In looking at a signal received from a mobile, if the signal looks weak (e.g., based on detected frame error rate (FER), for example), the base station may send a command to either increase or decrease mobile station transmit power. For example, a comfortable level of quality in a voice system may be possible with a FER of approximately 1%. If FER is much less than (<<) 1%, the mobile station may be wasting power, so the power control algorithm implemented at the base station may send commands to the mobile requesting the mobile to reduce the transmit power. For FER much greater than (>>)1%, the level of quality may be degraded, so the base station may send a command to the mobile to bring the mobile transmit power up in order to restore quality.
Typically, in order to effect power control at the base station, two power control loops may be utilized, which together provide what is referred to as ‘closed-loop power control’: inner loop power control and outer loop power control. In an exemplary CDMA communication system, for example, an inner loop power control algorithm (‘inner loop’ or ‘fast power control’), which may operate at a speed of 800 Hz, for example), may be used to adjust the power at the transmitter. Thus, a base station measures a received signal to noise ratio (Eb/Nt), also known as a signal to interference ratio (SIR), and compares the SIR value to a threshold. The threshold may be used by the inner loop to determine a specified quality of service (QoS) for power control. If the received SIR is too high (e.g., above the threshold), the base station transmitter may send a down power command to the mobile station, and vice versa where measured SIR is too low.
QoS may be representative of a number of different service requirements. For example, QoS may be indicative of providing guaranteed performance (e.g., such as a minimum or maximum data network throughput, a minimum delay requirement, a packet loss rate, and/or a packet download time, etc.) in a given network such as UMTS. A system or network such as a UMTS or CDMA system may be designed to support several quality of service (QoS) levels to allow efficient transfer of non real-time traffic (e.g., intermittent and/or bursty data transfers, occasional transmission of large volumes of data) and real-time traffic (e.g. voice, video), etc.
However, a communication path between base station and a mobile station is not often line of sight (LOS), and may be constantly changing due to the motion of the mobile station, or due to the mobile station's surroundings. For example, SIR changes may be caused by fast fading (like Rayleigh or Ricean fading), by shadowing (log-normal fading) and/or by changes in the interference level. Ideally, the received SIR should remain constant to enable a good reception of the reverse link signal (from the mobile to base station) without wasting transmit power at the mobile station.
Thus, the radio channel conditions between the base station and a mobile may be constantly changing. As the radio conditions change, the threshold may be adjusted in order to maintain the QoS of the radio link. The system that performs the function of adjusting the threshold (e.g., setting and adjusting the set point of the threshold) is referred to as the outer loop power control (‘outer loop’ or ‘slow power control’). Together with the inner loop, the outer loop forms the closed loop power control.
Outer loop power control may be designed to control the current link quality in terms of a bit-error rate (BER) or a block error rate (BLER), depending on requirements of the radio bearer service. Although the SIR is controlled by the inner loop power control, the received link quality may still change. As discussed above, these changes may be caused by variations in the multi-path delay profile (typical urban, hilly terrain, etc.), alterations in the speed of the mobile and/or modifications in the interference characteristics. The outer loop power control thus may adapt a ‘target SIR’ of the inner loop (such as by adjusting the set point of the threshold) so that the required link quality may be achieved.
In CDMA systems such as UMTS, there is thus a need for a power control mechanism or algorithm to overcome path loss effects, and to balance the quality of service (QoS) between the various user services. However, in certain situations, a CDMA system may become overloaded.
FIG. 1 is a graph to illustrate the overload problem in communication systems such as UMTS. Referring to FIG. 1, as load reaches 100%, the interference and transmit power for each mobile user may rapidly increase. In the high-load region, power control may not be effective, leading to unstable operation of the network. In a worst case scenario, a transmit power limit may have been reached, which could possibly lead to a drop of the connection, due to loss of synchronization between mobile and base station. To prevent an occurrence of this, efficient overload control algorithms are desired in CDMA networks such as UMTS.
Conventional methods for addressing power overload events or situations in wireless communication systems may include: (a) load or overload control; and (b) the aforementioned closed loop power control. Overload control may typically be employed in networks such as UMTS. Referring again to FIG. 1, it is desirable that the right-most region of loading should be avoided, because at this point, the network or system may become unstable due to a rapid increase in both the interference and transmit power. In FIG. 1, the region to be avoided may thus be referred to as an ‘outage area’.
In general, the following conventional methods may be employed to address the overload control problem in CDMA networks. One technique is referred to as Call Admission Control (CAC). CAC functionality attempts to avoid an overload situation by controlling the access of new users to the network or system. The basic CAC functionality thus operates so as to not admit (i.e. block) a new user into the system, if the load becomes greater than a given threshold (thradmit), as illustrated in FIG. 1, for example.
Another conventional method addressing overload may be referred to as ‘congestion control’ (ConC). Even with a properly functioning CAC routine, a wireless communication system may become overloaded due to the mobility of mobile users in the system. In such a case, ConC functionality may facilitate overcoming the overload situation. Referring again to FIG. 1, and in general, ConC methodology may interrupt (i.e. drop) an existing connection, if the load exceeds a second given threshold (thrdrop). The thrdrop threshold typically may be set to a greater value then thradmit. Accordingly, use of the above overload control methods (CAC and/or ConC) may lead to a relatively harsh reaction for users in a UMTS or CDMA system, if the system is in an overload situation.
Another radio resource management function related to overload control may be closed loop power control. Power control (PC) algorithms implemented at the base station may control the setting of the transmit power in order to (a) maintain system QoS within required limits, e.g., data rate, delay, BLER, etc.; and (b) to reduce and/or minimize interference, i.e., overall power consumption. PC may address propagation effects, like path loss (near-far-problem), shadowing (log-normal-fading) and fast fading (Rayleigh-fading, Ricean-fading), as well as the impact of the environment (delay spread, UE speed, etc.) on the wireless communication system.
FIG. 2 illustrates a block diagram of conventional closed loop power control (CLPC) in a base station transceiver of a wireless communication system. As discussed above, CLPC may include inner loop power control (ILPC) and outer loop power control (OLPC). As shown in FIG. 2, there is shown a block diagram of a transmitter side 210 of a user such as a UE and a receiver side 220 of a base station transceiver (hereafter the terms ‘base station’ and ‘NodeB’ may be occasionally interchanged).
The ILPC controls the transmit power settings of the transmitter side 210 in order to achieve a desired or given SIRtarget, which has been adjusted by the OLPC. Referring to FIG. 2, the basic function of ILPC may be as follows. At the receiver side 220 of the NodeB, the SIR of a given UE may be estimated (at 225) from a signal 222 received via receiver 221. The received signal 222 may be compared against SIRtarget at element 225. Based on the comparison, transmit power control (TPC) commands 226 may be generated. For example, if SIR<SIRtarget, the ILPC of a given NodeB may generate a ‘power up’ command to the UE that it is serving; if SIR>=SIRtarget, the ILPC of a given NodeB may generate a ‘power down’ command to the UE.
The TPCs 226 may be multiplexed by a suitable MUX 227 into a data stream 211 sent to the associated transmitting side 210 of the UE. The transmitter side 210 extracts the TPC commands 226 from the associated data stream 211 at a suitable DEMUX 212. The transmitter side 210 may adjust the transmit power of a power amplifier (PA) 217 therein based on the extracted TPC commands received from DEMUX 212 via line 216. For example, when a power up command has been received from receiver side 220, the transmit power of the UE may be increased by a given amount; after reception of a power down command, the transmit power may be decreased by a given amount. ILPC is thus a closed loop between the transmitter side 210 of the UE and receiver side 220 of the NodeB.
The OLPC controls the SIRtarget setting (threshold) for ILPC in order to fulfill the QoS requirement of the service, which may be given, for example, by a certain BLERtarget 230. Referring to FIG. 2, the basic function of OLPC may be as follows. Within the receiver side 220, the QoS of the service (e.g. the BLER) may be estimated (at 228) from the decoded signal 224 (decoded by decoder 223) and compared against a QoS target (e.g. given by BLERtarget). An adjusted SIRtarget for ILPC may be determined (see line 229) based on the comparison at 228. For example, when BLER>BLERtarget, OLPC increases SIRtarget, and when BLER<=BLERtarget, OLPC decreases SIRtarget. The revised SIRtarget (threshold) may be provided to ILPC. OLPC is also a closed loop, which primarily runs within the receiver side 220 of the NodeB.
In the uplink (‘reverse link’, mobile to base station) power control is performed for each mobile user, separately, while in the downlink (‘forward link’, base station to mobile) power control runs per physical channel. In current realizations, no specific power control action is performed when a power overload situation occurs. In an overload situation, conventional power control increases the transmit power to the transmit power limit, even if the desired or target BLER has not yet been reached or met. This may lead to unexpected droppings of existing connections, as the synchronization between transmitter and receiver has been lost.
FIG. 3 illustrates a conventional arrangement for uplink power control in a wireless communication system. System 300 may be embodied as a UMTS Terrestrial Radio Access Network (UTRAN) 300, for example. UTRAN 300 may include cell sites, called Node Bs 310 (base stations), which may serve one or more UEs 315, generally using a Uu interface protocol. A Node B 310 may contain radio transceivers that communicate, using lub protocol, with radio network controllers (RNCs) 320 and 325. Here, RNCs are shown as the controlling, or serving RNC (SRNC) 320 and a drift RNC (DRNC) 325 of the UTRAN 300. The SRNC 320 and DRNC 325 may communicate with each other using an lur protocol, for example.
UTRAN 300 may also interface with one or more core networks (CNs) 340 (only one being shown in FIG. 3 for simplicity). Although not shown for reasons of brevity, CN 340 may include mobile switching centers (MSCs), one or more Serving GPRS Support Nodes (SGSNs) and one or more Gateway GPRS serving/support nodes (GGSNs). SGSNs and GGSNs are gateways to external networks (not shown). In general in UMTS, SGSNs and GGSNs may exchange packets with mobile stations over the UTRAN 300, and may also exchange packets with other internet protocol (IP) networks, referred to herein as packet data networks (PDNs). External networks may include various circuit networks such as a Packet Switched Telephone Network (PSTN) or an Integrated Service Digital Network (ISDN) and PDNs.
As shown in FIG. 3, UTRAN 300 may be linked to CN 340 via suitable lu interfaces such as lu cs and lu ps (not shown for clarity), for example. Alternatively, UTRAN 300 may be linked to the CN 340 via back-haul facilities (not shown) such as T1/E1, STM-x, etc., for example. Ics, short for Interface Unit (Circuit Switched) interface, is the interface in UMTS which links the RNC with a MSC. Ips, short for Interface Unit (Packet Switched) interface, is the interface in UMTS which links the RNC with a SGSN.
According to the 3GPP UMTS standard such as 3GPP TS 25.401, V6.2.0 (2003-12), entitled “UTRAN overall description), the different uplink PC functions may be located in different network entities. FIG. 3 illustrates an example for the location of uplink control functions in a case of soft handoff between NodeBs 310. Soft handover is a call configuration whereby the UE is simultaneously connected to more than one cell. In this example, the UE 315 is connected to different NodeBs 310 and different RNCs 320 and 325, with each NodeB 310 connected to an associated controlling RNC 320/325 via the lub interface. If the controlling RNCs are different for the NodeB 310 involved in soft handoff, one RNC takes the part of SRNC 320 for controlling the connection, while the other acts as a DRNC 325. As shown, UE 315 data and control flows between RNCs 320/325 may be transmitted via the lur logical interface.
Conventionally, the ILPC function is located in each NodeB 310, thus providing separate ILPC loops between a given UE 315 and each NodeB 310. The 3GPP UMTS standard provides specified rules for the UE 315, in a case where the TPC commands from these separate loops are in conflict. The frame selector 322 may be adapted to select the most reliable data stream between the different uplink soft handover paths that reach the SRNC 320, and may be located in the SRNC 320. Also, OLPC functionality 324 may typically also be located in SRNC 320, as shown in FIG. 3, for example.
Accordingly, conventional power control implementations do not present an efficient way of addressing and handling overload conditions, particularly in the uplink. For example, for a short term overload, such as a few tenths of a second, for example, any action by the PC may be too harsh, adversely affecting users by blocking or dropping users as currently done by conventional overload control algorithms such as CAC and/or ConC.
Further, in the case where no action is done in response to an overload situation, the power control loops for a given number of users may reach corresponding limits, which could lead to unexpected and uncontrolled performance degradation for the service. In the extreme case, synchronization between a given UE 315 and its serving NodeB 310 may be lost and the call dropped.